The present invention relates to impregnated fired ceramic cores for use in investment casting of metallic materials and a method of increasing strength of such cores.
In casting hollow gas turbine engine blades and vanes (airfoils) using conventional equiaxed and directional solidification techniques, a fired ceramic core is positioned in an investment shell mold to form internal cooling passageways in the airfoil. During service in the gas turbine engine, cooling air is directed through the passageways to maintain airfoil temperature within an acceptable range.
The fired ceramic cores used in investment casting of hollow turbine engine airfoils typically have an airfoil shape with a quite thin cross-section trailing edge region. Such ceramic cores can be prone to distortion and loss of the required dimensional tolerance during core manufacture and subsequent steps of the investment casting process such as wax pattern injection about the fired core and steam autoclaving of the shell mold to selectively remove the wax pattern.
Green (unfired) ceramic cores typically are formed to desired core configuration by injection molding, transfer molding or pouring of an appropriate ceramic core material that includes one or more ceramic powders, a fugitive binder such as wax, polyproplylene, polyolef in, prehydrolized ethyl silicate, and other additives into a suitably shaped core die. After the green core is removed from the die, it is subjected to firing at elevated (superambient) temperature in one or more steps to remove the fugitive binder and sinter and strengthen the core for use in casting metallic material, such as a nickel or cobalt base superalloy. As a result of removal of the binder and fugitive filler material, if present, the fired ceramic core is porous.
Attempts have been made to further strengthen the fired, porous ceramic core. For example, the fired, porous ceramic core can be impregnated with an aqueous solution of a water-soluble phenolic formaldehyde resin followed by a 300-400 F. degree oven bake to set the phenolic resin. Use of the water-soluble phenolic formaldehyde resin solution as an impregnating medium is disadvantageous as a result of reduction of the impregnation strengthening effect imparted by the water-soluble resin in the presence of atmospheric moisture, such as water and steam, that may be present in the core manufacturing and foundry environment. That is, the strengthening effect imparted by the water-soluble resin is degraded in the presence of atmospheric moisture. The use of the water-soluble phenolic formaldehyde resin solution as an impregnating medium is also disadvantageous from the standpoint of presenting environmental and health concerns with respect to the formaldehyde resin. Cores impregnated with the water-soluble phenolic formaldehyde resin solution can exhibit dimensional distortion during the oven baking operation.
An object of the present invention is to provide an impregnated fired ceramic core and method of strengthening the fired core while overcoming the above-noted disadvantages.
An embodiment of the present invention provides an impregnated fired porous ceramic core for use in an investment shell mold in the casting of molten metals and alloys wherein the core is impregnated with an aqueous emulsion of a water-insoluble polymer followed by drying to remove the water.
The impregnated fired porous core pursuant to the invention exhibits a greater strength increase than achieved by the water-soluble phenolic formaldehyde resin impregnated core after oven baking, and the strength increase so imparted is more resistant to degradation in the presence of atmospheric moisture.
In a particular embodiment of the present invention, the water-insoluble polymer is selected from the group consisting of acrylic, styrene butadiene, polyvinyl acetate, styrene acrylic, vinyl acetate acrylic, vinyl-vinylidene chloride, epoxy, polyvinyl butyrol, polyurethane and other water-insoluble polymers.
A particularly preferred aqueous emulsion comprises about 10% to 50% by weight of an acrylic polymer and balance essentially water where the acrylic polymer is self cross-linkable. An even more preferred aqueous emulsion comprises about 15% to 30% by weight of the acrylic polymer and balance essentially water. The acrylic polymer preferably has a Tg (glass transition temperature) from 15 to 40 degrees C. The aqueous emulsion can include the addition of minor amounts of constituents to reduce foaming, enhance wetting, and/or improve polymer cross-linking.
The invention provides a fired, porous ceramic core for use in investment casting including water-insoluble polymer solids in pores of the core, the polymer preferably being present in an amount of about 0.2% to about 5% by weight of the core.